1. Field of the Invention
The present invention relates to a photovoltaic cell having a high conversion efficiency and reliability, which uses a metal layer free from reflectance reduction at near 800 nm inherent in aluminum. Also, the present invention relates to a photovoltaic cell having improved adhesion between a substrate and a metal layer.
2. Description of the Related Art
Single-crystal and polycrystalline silicon have been primarily used in solar cells. Since fabrication of solar cells consumes much energy and time in the growing step of these silicon crystals and requires complicated succeeding steps, cost reduction due to mass production is hardly achieved. Recently, thin film semiconductor solar cells using compound semiconductors, such as amorphous silicon (hereinafter referred to as a-Si) and CdS.multidot.SuInSe.sub.2, have been vigorously developed. Since a semiconductive layer can be formed on an inexpensive substrate made of glass, stainless steel or the like according to demand by a relatively simple fabrication process, thin film semiconductor solar cells have advantages in material and production costs. Thin film semiconductor solar cells, however, have lower conversion efficiencies relative to crystal silicon solar cells and lack reliability for long term use. Therefore, they are not used in earnest. Various ideas have been disclosed to improve thin film semiconductor solar cell performance.
One method for such improvement relates to a back metal layer provided on a substrate surface in order to enhance the reflectance of light on the substrate surface and effectively use incident light, in which sunlight not absorbed in the semiconductor thin layer is reflected back towards the semiconductor thin layer so as to be absorbed. Since short wavelength components of incident sunlight have already been absorbed in the semiconductor thin layer, high reflectance is required for longer wavelength components. A critical wavelength for high reflectance depends on the absorption coefficient and thickness of the semiconductor thin layer. When sunlight is incident on the semiconductor thin layer through the substrate side of a transparent substrate, a metallic electrode having high reflectance, e.g. silver (Ag) or copper (Cu), is preferably formed on the semiconductor thin layer surface. FIG. 2 is a graph for comparison of reflectance of Ag, Al, Cu and Ni films each having a thickness of 2,000 .ANG.. When sunlight is incident on the semiconductor thin layer surface, a similar metal layer is preferably formed on the substrate surface before forming the semiconductor thin layer. A transparent conductive layer intercalated between the metal layer and the semiconductor thin layer further enhances the reflectance due to multiple interference effects. Use of the transparent conductive layer also causes increased reliability of thin film solar cells. Japanese Examined Patent Publication No. 60-41,878 discloses that use of a transparent conductive layer can prevent alloying of a semiconductor and metal. U.S. Pat. Nos. 4,532,372 and 4,598,306 disclose that use of a transparent conductive layer having a moderate resistance can prevent excessive current between electrodes when short-circuiting occurs in the semiconductive layer.
Another method for enhancing conversion of thin film solar cells relates to a fine uneven structure or texture of the solar cell surface and/or interface with the back metal layer. With such a texture, sunlight scatters on the solar cell surface and/or interface with the back metal layer, is trapped in the semiconductor (light trapping effect), and thus is effectively absorbed by the semiconductor. When using a transparent substrate, a transparent electrode having a fine uneven texture surface, made of tin dioxide (SnO.sub.2) or the like, is preferably formed on the substrate. When sunlight is incident on the thin film semiconductor surface, a back metal layer having a fine uneven textured surface is preferably used. M. Hirasaka et al. discloses that a back metal layer having a fine uneven textured surface can be formed by depositing aluminum under a regulated substrate temperature and deposition rate (Solar Energy Materials 20(1990) p. 99-110). FIG. 3 is a graph illustrating an increase in absorption of incident light due to use of such a textured back metal layer, wherein curve (a) is a graph illustrating spectral sensitivity of an a-SiGe solar cell using specular silver as a metal layer, and curve (b) is a graph illustrating spectral sensitivity of a solar cell using textured silver. FIG. 3 demonstrates that light of near 800 nm is not effectively used in the a-SiGe semiconductive layer, and therefore, use of a back metal layer having high reflectance for light of near 800 nm further enhances conversion. FIG. 2 demonstrates that silver and copper have high reflectance over the entire wavelength region between 700 and 1,000 nm, whereas aluminum has a local minimum reflectance at near 800 nm. Therefore, silver and copper, having high reflectance at 800 nm, are most suitable for the metal layer.
However, it is known that electrochemical migration occurs in these metals and in particular in silver. Electrochemical migration (hereinafter referred to as merely "migration") refers to a phenomenon that occurs when a metallic foil, plating or paste is used in a contact state with a hygroscopic or hydrophilic insulating material in a high humidity environment while applying direct current; the metal forms electrically conductive paths as a result of dendritic or speckled growth of electrolysis products on and in the insulating material. Some metals require additional factors for electrolysis. For example, experimental results illustrate that silver (Ag), copper (Cu) and lead (Pb) require distilled water and an electric field for migration (Ag deposits dendritic crystal at an extremely high rate), gold (Au), palladium (Pd) and indium (In) further require halogen ions, and aluminum (Al), nickel (Ni) and iron (Fe) require special conditions other than the above-mentioned factors.
Interelectrode short-circuiting due to migration is a problem to be solved in solar cells used in various environments for long time periods, for example, a solar cell used outside in a high temperature, high humidity environment. Since a single solar cell has a low output voltage, a plurality of submodules (modulated thin film semiconductor solar cells) connected in series are generally used. When the solar cells are partly covered with fallen leaves, the output current of the covered submodules drastically decreases relative to the uncovered submodules and the internal impedance increases. As a result, an output voltage from uncovered submodules is applied to the covered submodules. A condition causing migration, that is, inverted bias impression at high temperature and high humidity, is established, resulting in interelectrode short-circuiting and submodules breakage. Use of high reflectance Ag or Cu as the back metal layer further promotes such migration. Since Al, highly resistive to migration, has a wavelength region of low reflectance at near 830 nm, use of Al does not achieve high conversion compared to Ag and Cu.
A combination of a back metal layer comprising a metal layer and a transparent conductive layer with a textured structure is also effective. U.S. Pat. No. 4,419,533 discloses a transparent conductive layer formed on a metal layer with a surface textured structure. Formation of a transparent conductive layer with a textured structure on a specular metal layer will also be feasible. Such a combined technique is expected to significantly increase conversion of solar cells.
Adhesion between the substrate and the metal layer deteriorates during use of the photovoltaic cell in a high temperature, high humidity environment, a high chloride environment on the sea or seaside, and a hot temperature environment in a desert. Adhesion between the metal layer and the transparent conductive layer also deteriorates during use of the photovoltaic cell in such environments.